Low tension spark plugs



Oct. 27, 1959 G. D. SUTER 2,910,606

' Low TENSION SPARK PLUGS Filed Sept. 26, 1955 INVENTOR. Gzoaas D.Su-reR wuw ATTORNEY LOW TENSION SPARK PLUGS George D. Suter,Thiensville, Wis, assignor, by mesne assignments, to the United Statesof America as represented by the Secretary of the Army ApplicationSeptember 26, 1955, Serial No. 536,329

1 Claim. (Cl. 313-431) This invention relates tospark-plugs; moreparticularly to the creepage gap type of spark plugs used in a lowtension ignition system for an internal combustion engine.

Electrodes in plugs of this type are ordinarily subject to rapid erosionbecause the spark is of high amperage and energy level. This tends tomaterially decrease the useful life of the plug.

It is the object of this invention, therefore, to provide a spark plugof this type which will have a prolonged useful life within which itsoriginal firing characteristics are maintained.

This object is obtained by utilizing electrodes which are highlyresistant to erosion, supporting such electrodes so as to have properheat dissipation, arranging the sparking surfaces so that erosion willnot adversely increase the gap, and using glass insulation in said gapbonded to said surfaces.

In general the sparking surfaces are a part of an electrode assemblywith a glass insulation gap separating, but bonded to, the sparkingsurfaces. This assembly is relatively small and is inserted within theend of the steel shell of the plug from which it receives its mechanicalsupport and heat dissipating properties. Thus the volume of specialalloy is minimized. The encircling gap causes the erosion to take placeradially inwardly without harmfully increasing the gap between sparkingsurfaces.

The features of the invention described herein which are considered newand useful and, therefore, patentable are set forth in the claim below.The details and scope of the invention itself can best be understood byreading the specification and analyzing the drawings, in which:

Fig. l is a sectional elevation view through the center of the lowtension spark plug embodying this invention; and

Fig. 2 is an enlarged bottom View of the electrode assembly embodyingthis invention with parts broken away and shown in section to illustratethe relationship between the electrodes and the layers of insulation.

The particular adaptation of this invention shown by the drawings isincorporated in a spark plug of standard design, except for noveldifferences set forth hereinafter. A steel center terminal or stud 10 issealed in place within a high alumina porcelain core 12. A nickel alloyconnector 14 is connected to the terminal 10 and extends beyond theinner end of core 12. The inner end of this connector is slotted toprovide a forked end 16 which makes contact with the electrode assembly.The core 12 is held within a threaded steel shell 18 against a gasket 20by an aluminum seal 22 (see my Patent No.

2,632,431, dated March 24, 1953). The threaded end of the shell 18 has abore with an inwardly facing shoulder 26 at its outer end. The electrodeassembly hereinafter described is fixedly mounted in this bore againstsuch shoulder with the forked end 16 connected to such assembly.

The principal feature of this invention resides in a United StatesPatent sparking surfaces.

2,910,606 Patented Oct. 27, 1959 solid electrode assembly consisting ofa radially outer sleeve-like ground electrode 28 and a center electrode30. The ground electrode 28 has an inturned outer flange 32 which formsan inwardly facing seat for a preformed high alumina porcelain bushing34 forming part of such insulation. The center electrode 30 has a solidcylindrical body 36 which fits snugly Within the bushing 34. An outwardprojecting neck 38 on such body is center drilled to have a press fit onthe forked end 16. Near the inner end such body has a larger diameter toform an outwardly facing shoulder 40 which engages the inner end of thebushing 34 to properly position the electrodes. At its inner end thebody 36 has a radial flange 42 which has a diameter the same as that ofthe ground electrode 28. The sparking surfaces are the inward face andcor .ner on the ground electrode 28 and the outward face and corner ofthe flange 42. The space between the sparking surfaces is filled withglass 44 bonded 'to such surfaces and :forming another .part. of suchinsulation. Both electrodes are made of a metal such as a copper-.tungsten alloy, which is strong mechanically and resistant to sparkerosion. When the spark plug is first placed in operation the sparkcreeps over the surface of the glass 44- from corner to corner. Withcontinued usage the face of the glass will erode inwardly at a very slowrate exposing the inward and outward sparking surfaces to which theglass had been bonded. However, the special alloy material so resistserosion that those sparking surfaces do not recede from each otherenough to make any change in the gap which effects the jumping of thespark. The bond between the insulation and the sparking surfacesprevents the sparking from creeping in between. Thus the plug can beused until the glass is eroded back to the radially inward edge of thesparking surface.

To make the electrode assembly, the bushing 34 is assembled in theground electrode 28 and the body section 36 of center electrode 30inserted part way into bushing 34. Insulating glass in powder form isthen compacted around body 36 and between the sparking surfaces. Theassembly is then placed in a fixture and heated until the glass becomesplastic after which body 36 of center electrode 30 is forced intobushing 34 until shoulder 40 comes to rest on the end of bushing 34. Thedimensions of the components are such that upon cooling a gap of .005/.010 is established between the The glass insulation has several veryimportant characteristics. They are: (1) very high electricalresistance; (2) very high resistance to spark tracking and heat shock;(3) a coefficient of thermal expansion substantially the same as that ofthe electrode material; and (4) intimate bond with the sparking surfacesforming the gap. It should be noted that, in contrast to previousso-called creepage gap spark plugs, the insulation between the gapemploys no semi-conducting material either as a surface coating orembedded particles. It has been found that such coatings and materialsare erratic in behavior during the life of the plug under varyingservice conditions. In the described plug the available firing voltageis sufficient to jump the (.005 .010) gap with no need for lowering itsresistance. After the electrode assembly is completed it is assembledinto shell 18 as follows: the shell is heated to approximately 500 F.and the electrode assembly is forced with a light press fit into theshell bore. Upon cooling the shell shrinks slightly and holds the groundelectrode 28 firmly. Thereafter the sub-assembly of stud 10, connector14 and core 12 are assembled to the shell 18 in the usual manner. Theforked end 16 fits into the drilled neck 38 and keeps constant contactby reason of its inherent resiliency. The overall coefficient ofexpansion 3 of the assembly is less than that of the shell 18. The sparkplug shell 18, being a heat dissipating surface, will remain cooler thanthe ground electrode 28 and continue to hold the assembly firmly inplace.

The spark plug described above has the following advantages: (1) thenarrow gap of the electrode assembly can be accurately set beforeinsertion into the shell of the spark plug; (2) the coeflicients ofexpansion are such that the thermal stresses are avoided in criticalareas and the assembly is continuously held securely'within the shell atall temperatures; (3) the costly materials of the electrodes of theassembly are such that the spark erosion is minimized and such assemblylessens the amount of such materials in each plug; and (4) proportioningof all materials can be obtained to get the best thermal conductivity inthe areas where heat dissipation is required.

Although only one specific embodiment of this invention has been shownand described, many modifications are possible. This invention,therefore, is not to be restricted except in so far as is necessitatedby the prior art and by the spirit of the appended claim.

-I claim as my invention:

A spark plug including a steel shell with an internal bore at its lowerend, said shell being provided with a shoulder in said bore, a connectorhaving a forked lower end carried by and insulated from said shell andextending into said bore; an electrode assembly comprising a hollowcylindrical ground electrode of copper tungsten alloy having an inturnedflange at its outer end, said ground electrode being received in saidinternal bore with said inturned flange in abutment with said shoulder,a bushing of high alumina porcelain insulating material seated insidesaid electrode against said inturned flange, a center electrode ofcopper tungsten alloy having a body with an outwardly facing shoulderfitted within said bushing with said shoulder in contact with saidbushing, said center electrode having a radially projecting flange onthe inner end thereof facing and spaced from the inner end of and of thesame outside diameter as said ground electrode, said center electrodehaving a neck at the outer end thereof receiving said forked lower endof said connector, and insulating means between said radially projectingflange and said inner end of said ground electrode consisting of glassbonded and fuzed thereto.

References Cited in the file of this patent UNITED STATES PATENTS

